[Bug preprocessor/64831] New: Preprocessing # operator with C++11 user-defined literal produces bad output

[holger.grund@ix-n.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64831

            Bug ID: 64831
           Summary: Preprocessing # operator with C++11 user-defined
                    literal produces bad output
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: holger.grund@ix-n.net

$ cat t.cpp
#define S1(a) S2(a)
#define S2(a) #a

void foo()
{
  S1("bla"_udl);
}

$ c++ t.cpp -E -P
void foo()
{
  "\"bla\"_udl";
}

$ c++ t.cpp -E -P -std=c++11
void foo()
{
  ""bla"_udl";
}

Feeding the resulting output from c++11 mode into the compiler gives an error:
$ c++ t.cpp -E -P  | c++ -c -xc++ -std=c++11 -
$ c++ t.cpp -E -P -std=c++11 | c++ -c -xc++ -std=c++11 -
<stdin>: In function ‘void foo()’:
<stdin>:3:8: error: unable to find string literal operator ‘operator""bla’

This breaks https://github.com/icecc/icecream for remote compilation.